Sorry I don't have Tableau 10 yet (still stuck in 9.3 so I can publish to our server), so I wasn't able to download your workbook.
However, I've created an example with Superstore data that looks at two measures, and then a ratio of them day-by-day. You'll want to change the field "Prof/Quantity" to a difference instead of ratio.
If you just wanted the difference between the two, I think if you created a calculated field that was something like: (sum[OrdersPlaces]-sum[OrdersShipped]), and then put that in the columns with date on the X axis, you should see that trend.
Hope this helps!
diffovertime.twbx 1.2 MB